kitty: fix diff.conf being generated regardless of user config

'cfg.diffConfig' is an implicit submodule with sub-options that carry
their own defaults, so it never actually equals `{ }` - the previous
`cfg.diffConfig != { }` check was always true. This caused
'kitty/diff.conf' to be written (and clobber any existing unmanaged file)
even when the user configured nothing under `programs.kitty.diffConfig`.

modules/programs/kitty.nix

@@ -433,7 +433,7 @@ in
       '';
     };
 
-    xdg.configFile."kitty/diff.conf" = mkIf (cfg.diffConfig != { }) {
+    xdg.configFile."kitty/diff.conf" = mkIf (cfg.diffConfig.extraConfig != "") {
       text = ''
         # Generated by Home Manager
         # See https://sw.kovidgoyal.net/kitty/kittens/diff/